Rune,
"Out of the blue" output transistor failures have been happening in SS PA-s.
There are lots of discussions of the possible reasons on the various forums,
ranging form overdrive, thermal issues to transients in the output circuits,
etc. Nobody knows for sure yet.
One possible source is lightning or other high voltage discharge events,
which could be made worse by a high Q antenna. Against that, you should have
gas discharge (or spark-gap) protection devices on your feed-line.
Otherwise, if your antenna is matched to the coax, the voltage on the coax
should be determined by the power and the impedance of the coax (about 400
Vpeak at 1500 W on 50 Ohms). If the SWR is not 1:1 the voltage will be
higher: about 800 Vp at infinite SWR. If your PA has a 12:1 output
transformer, the transistors will see 67 V, which should be safe. "Narrowly
resonant" antennas, when matched, just have narrower low SWR ranges. Think
of the antenna (and matching circuit) as a transformer: It transforms the
voltages and currents on the feed line into crazy high voltages (and zero
currents) at the tips of the antenna. It is the PA driving the antenna, and
not the other way around.
The best protection is to cover everything: the sequencer you have installed
will help by preventing hot-switching or switching transients. Lightning
protection protects against much more than just direct hits. You should also
have something that prevents overdriving your amp.
GL and 73,
George,
AA7JV/C6AGU
On Wed, 13 Jan 2021 12:13:25 +0100
Rune Øye <rune.e.oye@gmail.com> wrote:
Hi all and HNY.
I have been told that Oscillation in Narrow tuned antennas can occur and
make damage to your equipment, radio or amplifier. It is really the first
time I have heard about this potential issue. On 160m band, I use a
Titanex HD with a matching unit, in fact it is 27.5 meter vertical with a
big loading coil at the feed point. 60 ground radials approx. 80 to 100
feet long and two ground rods. The antenna is perfectly matched at 1826Khz.
Reason for asking is that I have burned the final transistor in my radio
and the final FET`s in my PA is broken. It all happens “out of the blue”.
Since I now get a bit worried about this issue, I have installed a
sequencer to be sure that my Power Amp, and all my switches and boxes are
in time / sync so hope I have done something useful. Oscillation in Narrow
band tuned antenna, is this a common issue, how do I avoid it! tune the
antenna a bit out of resonance!
This is really the first time I have heard about this issue…… any
suggestion please.
Thanks,
73
Rune LA7THA
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